Sensory hair cell loss is the leading cause of deafness in humans. The mammalian cochlea cannot regenerate its complement of sensory hair cells and thus at present, the only treatment for deafness due to sensory hair cell loss is the use of prosthetics such as hearing aids and cochlear implants. Strategies for hair cell repair that involve either stimulation of quiescent cells in the mature organ of Corti, or which involve transplantation of progenitor cells able to differentiate into hair cells will require markers to identify such cells, as well as a detailed understanding of hair cell precursor biology. However, progenitor cells that give rise to hair cells of the mammalian inner ear are currently uncharacterized, largely because no definitive makers are available for their identification. We propose a pilot project to develop molecular genetic markers that identify unique subpopulations of otic epithelial progenitors. To accomplish this, we will use an important new resource (The Gensat Project) to identify markers of sub-populations of cells in the developing inner ear. This is a collection of transgenic animals that harbor bacterial artificial chromosomes (BACS) as transgenes and which express Green Fluorescent Protein (GFP) under the control of a single gene contained in the BAC. In each transgenic animal, GFP is expressed in a unique sub-population of cells dictated by the expression pattern of the gene in question. We have identified current BAC Transgenic Collection, with a list of genes reported in the literature to be expressed in the inner ear has identified twenty-three genes expressed in the inner ear. To explore the usefulness of the BAC Transgenic collection, we propose to analyze four BAC transgenic animals chosen from among this group, based on their potential to add to our knowledge of sensory hair cell progenitors. In Specific Aim 1, each transgenic line will be characterized developmentally for expression of GFP in otic sub-populations each sub-populations will be purified by Fluorescence Activated Cell Sorting. In Specific Aim 2, GFP-expressing sub-populations will be assayed for their ability to differentiate into hair cells using a newly developed dissociated culture system for sensory epithelium. In the Specific Aim 3, the same purified otic epithelial sub-populations will be profiled using micro-arrays to provide initial information about gene expression networks in the developing inner ear.